Disaster strikes the Jacksons' trekking stable when Mr Jackson has a serious accident and ends up in intensive care. If their trekking business can't carry on, the family will be ruined, so Frances and Louisa and their friends help out. On their very first trek, a mystery boy under an assumed name is booked to ride by his school.
Who is he, and why is he there? And why is he so terrified of the men who call themselves his uncles?
As the trek winds over the moors, triumph and disaster follow them.